The present invention relates to an optical disc system in which information is recorded on a recording medium or recorded information is reproduced therefrom, by using optical means, and more particularly to a beam access apparatus in such a system.
In an optical disc system in which a laser beam from a light source is modulated by a pulse signal from an external information source and is recorded in a binary manner onto a recording medium on the surface of a disc, or information already recorded is read out from the medium, guide grooves of the optimum width for the light spot and the optimum depth for the wavelength of the laser beam are usually provided on the recording medium, and tracking position control is conducted so that the minute light spot is positioned precisely on a guide groove. This condition is called the track-follow mode, in general, in which the positioning operation for the light spot is performed to make it follow accurately the guide groove (hereinafter called a track) on which the light spot is positioned, whereby the recording of information or the reproduction thereof is performed.
When information is to be recorded on or read from a track other than the track on which the light spot is positioned, the light spot must be moved to the target track. When moving the light spot between tracks separated by a long distance, it is usually transferred from the current track to the target track by using both a head access mode in which the whole of the optical head is first transferred in the radial direction of the disc by the head actuator to position the light spot roughly in the vicinity of the target track, and a lens access mode in which the objective lens in the optical head is then moved track by track in the radial direction of the disc by the lens actuator to position the spot on the target track by a repetition of the track-by-track transfer thereof. In those cases where the light spot must be moved for a short distance, the light spot is moved from the current track to the target track using the lens access mode alone. After the light spot is moved to the target track in this way, it is made to follow that track so that it is positioned thereon in the track follow mode described previously, and thus information is recorded on or read from the required track recorded information is reproduced therefrom.
In the head access mode, however, it is necessary to generate large accelerations and decelerations of the head actuator in order to move the optical head at high speed. When the optical head is moved at such a large acceleration and deceleration, the relative position of the objective lens in the optical head is displaced to a large degree by the acceleration and deceleration, since the objective lens is usually coupled to the head by a spring or the like. In particular, damped vibrations of the objective lens are induced in the tracking direction thereof by deceleration after the optical head is positioned roughly, and thereby damped vibrations of the light spot are also induced in the tracking direction. When damped vibrations occur in this way, it is impossible to position the light spot accurately until damped vibrations are reduced to some extent, and thus the positioning of the light spot on the required track takes long time. Moreover, the large displacement of the objective lens due to this acceleration and deceleration has an adverse effect on the optical system, and thereby accurate focusing control is probably impossible.
In the track-follow mode and the lens access mode, the optical head is positioned by the head actuator in response to an optical head position signal obtained by a position detector provided externally, and the light spot is positioned in response to a signal indicating the relative positions of the light spot and the tracking guide groove, by a tracking actuator driving the objective lens in the optical head. Since the optical head and the objective lens (and with it the light spot), are positioned separately, the positioning information on the objective lens does not influence the positioning of the optical head. As a result, any eccentricity in the tracking guide groove or shift in the position of the objective lens in the optical head due to the track jumping, for instance, causes distortion of a spring supporting the objective lens, and the spring force which is proportional to the distortion has adverse effects, as interference, on the tracking control system. Concretely, a lack of stability in the track jumping of objective lens (the light spot), and an increase in the positional offset in tracking control, etc., can be included as adverse effects. In extreme cases, the target track can not be reached, since a relatively large distance track-jump is impossible, and the increase in the positional offset reduces the reliability of the reproduction of recorded information.
Furthermore, in an optical head in which tracking control is conducted by moving the objective lens alone in the tracking direction, the movement of the objective lens causes the optical axis of the light reflected from the recording medium to shift relative to that of the incident light, causing a shift of the optical axis to a track position error detector. Thereby an offset is caused in the detection of errors in the position of the track, the accuracy of positioning in the tracking is reduced, and the reliability of the reproduction of recorded information is also reduced.